1. Field of the Invention
The invention relates to a method of producing a powder magnetic core and to a method of producing a magnetic core powder therefor.
2. Description of Related Art
There are many articles that electromagnetism, e.g., transformers, motors, generators, reactors, speakers, induction heaters, various actuators, and so forth, in our surroundings. For example, the stator core and the rotor core in a motor and the reactor core in a reactor are mostly made from powder magnetic cores made by the compacting of resin-coated soft magnetic powders. Due to this application of a resin film to the particle surfaces, such a soft magnetic metal powder for powder magnetic core fabrication suppresses the appearance of iron losses by establishing insulation for the powder and hence insulation for the powder magnetic core itself.
The methods used to coat magnetic powders can generally be divided into wet methods and dry methods. For example, Japanese Patent Application Publication No. 2008-303443 (JP-A-2008-303443) discloses the production of a powder magnetic core by bringing a soft magnetic powder into contact with a coating treatment solution prepared by dissolving a silicone resin in methanol; thereafter drying the soft magnetic powder to form a silicone resin film on the particle surfaces; and subsequently compacting the soft magnetic powder to form a powder magnetic core. This wet method, which employs a solvent to form a silicone resin film, can form a uniform silicone resin film on the surfaces of the magnetic particles. However, it requires a step of drying off the solvent and also requires the disposition of a vacuum device for degassing and thus inevitably entails increased costs from both a process standpoint and an equipment standpoint. The execution of a continuous magnetic powder coating process is also problematic.
In order to avoid the problems described above for wet methods, attention has focused on powder magnetic core production methods that utilize a dry process that does not employ a solvent. Japanese Patent Application Publication No. 2008-270539 (JP-A-2008-270539) and Japanese Patent Application Publication No. 2009-259939 (JP-A-2009-259939) disclose powder magnetic core production methods including a mixing step, in which a resin powder formed from a thermosetting silicone resin is mixed with a magnetic powder having an insulating film, e.g., a silica film, on the particle surface; a compacting step, in which the mixed powder provided by the mixing step is compacted in hot state; and a heating step, in which the compact provided by the compacting step is heated to a high temperature state at which the silicone resin cures. In addition, the compacting step includes a heating step, in which the mixed powder filled into a die is heated to bring it into hot state; and a compression step, in which the mixed powder, while residing in a state in which the resin powder has been softened due to the heating step, is compacted. This hot state denotes a high temperature environment in which the resin powder does not undergo a complete condensation polymerization. In this Specification, “hot compacting” refers to a method of obtaining a compact via a compacting step in which compacting is performed in hot state temperature environment.
The methods disclosed in JP-A-2008-270539 and JP-A-2009-259939 can produce a powder magnetic core without using a solvent. However, these methods, by their very nature, include just the compacting of a mixture of a resin powder and a soft magnetic powder that has been insulated with a silica film. As a consequence, the role of the resin in the powder magnetic core resides more in strengthening the powder magnetic core through particle-to-particle bonding than in the insulation of the soft magnetic powder. Accordingly, when use is made of a soft magnetic powder that has not been insulated with, e.g., a silica film, it is thought that, for example, large losses will occur without the ability to obtain a thorough coating of the particle surfaces by the resin, and the magnetic properties will decline.
Moreover, a powder magnetic core is fabricated in each of the examples given in JP-A-2008-270539 and JP-A-2009-259939 using not more than 0.3 mass % resin powder with reference to the mixed powder as a whole. It is stated in JP-A-2008-270539 that when the resin powder is incorporated at 0.2 mass %, the compact provided by compacting in hot state can be removed from the die using a low decompacting pressure without producing, for example, galling with the die. The inventors have in fact confirmed that a powder magnetic core having the desired magnetic properties and strength and also free of problems with its appearance is obtained when the resin powder is incorporated at 0.2 mass %.
However, it was also discovered that a powder magnetic core having a normal appearance is not obtained when the same procedure as described in JP-A-2008-270539 or JP-A-2009-259939 is used to produce a powder magnetic core that has a relatively large resin powder content, as is used, for example, in reactor cores. The abnormalities in appearance included, for example, roughening and cracking of the surface of the powder magnetic core, chipping at angles of the powder magnetic core, and lamination. These abnormalities in appearance pose a number of risks; for example, they can lead to breakage, they can prevent use due to their effect on the dimensional accuracy, and, even when they do not have a direct influence on the magnetic properties, they can lower the reliability.
It was further discovered that the filling behavior by the mixed powder is impaired when a mixed powder containing relatively large amounts of resin powder is filled into a die that has been preheated to hot state; for example, the particles may aggregate or coalesce with one another and the resin powder may melt bond to the surface of the die. This impaired filling behavior is thought to be connected to the impaired compacting behavior noted above.